Feb. 1, 1868.] 



HAKDWICKE'S SCIENCE-GOSSIP. 



25 



IKBITABILITY AND SENSATION. 



^^^ERHAPS the 

 most difficult 

 problem which 

 the student of 

 Natural His- 

 tory can set 

 himself to 

 solve, is that which has re- 

 ference to the heading of this 

 paper; I mean the limits — 

 jwrea W ^ limits there be — between 



A- fJj&NrT w ^ l ' lc muscu l ar movements of 

 an animal and those of a vege- 

 table ; between what we call 

 sensation in the former and 

 irritability in the latter. 



Are Diatoms, Desmids,Vol- 

 vocinea), Oscillatorice, and a 

 host of similar organisms, 

 really vegetables ? If so, how 

 are we to distinguish be- 

 tween their movements and 

 those of animals of apparently 

 as humble a conformation ? There may not be much 

 rationality in the swinging motion of an Oscillatoria ; 

 but what shall we say of the Zoospores of the 

 Algae, which lash the water with their cilia precisely 

 as do the Infusoria ? or what, again, of many of 

 the Diatoms (Navicular, Pinnularise, and other free 

 species) which are seen to traverse the field of the 

 microscope with so much ease and grace ? Is all 

 this mere mechanical irritability ? If the supposed 

 discovery of their movements being due to the 

 contraction and expansion of an envelope of sarcode, 

 be confirmed, we shall have to change our ideas 

 very considerably, I expect, regarding this wondrous 

 family, which has so often been obliged to shift its 

 quarters from one kingdom to the other. 



Is the question nearer a solution when we leave 

 this debatable land and pass up to plants and 

 animals of a higher organization ? I scarcely think 

 so. It is not worth while, perhaps, to bring for- 

 ward, as an instance of voluntary motion, the perse- 

 verance of a plant in turning leaf and branch in 

 No. 38. 



the direction of the light ; or its unvarying efforts to 

 spread its rootlets only where the ground is moist 

 and soft. This seems wonderfully like perception ; 

 but in point of fact it is simply the effect of 

 growth. A nearer approach to real sensation is 

 seen in the power possessed by many plants of 

 closing leaves and flowers at given periods or under 

 certain circumstances. Every night the common 

 clover folds its leaflets together ; and the daisy 

 draws its ray florets close around the disk, so as 

 to form a canopy and protection. Indeed, so re- 

 gularly do many plants shut and open their petals 

 at different periods of the day, that a "Eloral 

 Clock " has been constructed denoting the hour by 

 the expansion or contraction of certain ^flowers. 

 Others, again, are dependent on the state of the 

 atmosphere for the unfolding of their petals. The 

 bindweed, the duckweed, the scarlet pimpernel, 

 and some others, invariably close their petals, as if 

 to shelter the orgars of fructification, before a 

 threatened storm ; indeed, so accurately does this 

 last-named plant perform this function, as to have 

 gained for itself the name of the "Poor Man's 

 Weather-glass." Eew botanical students are 

 ignorant of the movements in the leaflets of Des- 

 modium gyrans ; and fewer still, probably, have failed 

 to witness the graceful manner in which the sensi- 

 tive plant {Mimosa pudica) shrinks at the slightest 

 touch. A plant nearer home, and therefore more 

 accessible — the berberry — exhibits a similar pheno- 

 menon as distinctly, though not so conspicuously, 

 as does the Mimosa. In this shrub, the stamens, in 

 their normal condition, lie back closely pressed 

 against the yellow petals. No sooner, however, is 

 a filament touched in the slightest degree, as it 

 frequently must be by the legs of bees and flies, 

 than it flies upwards with a distinct jerk, and 

 presses against the stigma. Strange to say, the 

 plant loses this property if exposed to the vapour 

 of chloroform for a short time ; exactly as an animal 

 under similar conditions loses its function of sensa- 

 tion, to be restored when the exciting cause is 

 removed. 

 But perhaps the most animal-like of all these 



